Radiosurgery is a minimally invasive procedure that delivers high doses of ionizing radiation, in mono- or hypo-fractionated treatments, to destroy tumors or focal areas of pathology. The radiation dose has to optimally fit the tumor shape, while reducing the damage to collateral organs. The identification of the targeted lesion and its surrounding critical tissues is typically performed in a three-dimensional (3-D) space relative to the patient's reference frame during the pre-operative lesion identification phase. During the pre-operative planning phase, a conformal dose volume is sculpted around the target while minimizing the dose delivered to adjacent healthy tissues. This may be achieved using a combination of beam positions whose relative weights or dose contributions have been scaled to volumetrically shape the dose accordingly. In the model known as forward planning, the user manually specifies the desired weight of the various beams. The inverse planning method utilizes an algorithm to automatically calculate the optimum combination of beams and weights based on user-defined dose constraints to the target and healthy tissues.
Another method for tumor treatment is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source is changed, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low. The amount of radiation utilized in radiotherapy treatment sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centi-Gray (cGy)), short treatment times (e.g., 10 to 30 minutes per treatment) and hyperfractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted by the magnitude of the radiation.
During radiation treatment, a patient can change his or her position or orientation. In addition, pathological anatomies (e.g., tumor, legion, vascular malformation, etc.) may move during treatment, which decreases accurate target localization (i.e., accurate tracking of the position of the target). Most notably, soft tissue targets tend to move with patient breathing during radiosurgical treatment delivery sessions. Respiratory motion can move a tumor in the chest or abdomen, for example, by more than 3 centimeters (cm). In radiation treatment, accurate delivery of the radiation beams to the pathological anatomy being treated can be critical, in order to achieve the radiation dose distribution that was computed during the treatment planning stage.
One conventional solution for tracking motion of a target utilizes external markers (e.g., infrared emitters) placed on the outside of a patient (e.g., on the skin). The external markers are tracked automatically using an optical (e.g., infrared) tracking system. However, external markers cannot adequately reflect internal displacements caused by breathing motion. Large external patient motion may occur together with very small internal motion. For example, the internal target may move much slower than the skin surface.
Another conventional solution for tracking motion of a target involves the use of implanted fiducials. Typically, radiopaque fiducial markers (e.g., gold seeds or stainless steel screws) are implanted in close proximity to, or within, a target organ prior to treatment and used as reference points during treatment delivery. Stereo x-ray imaging is used during treatment to compute the precise spatial location of these fiducial markers (e.g., once every 10 seconds). However, internal markers alone may not be sufficient for accurate tracking. Furthermore, the tracking of internal fiducial markers can be difficult for the patient, because high accuracy tends to be achieved by using bone-implanted fiducial markers. The implanting of fiducial markers in bone requires a difficult and painful invasive procedure, especially for the C-spine, which may frequently lead to clinical complications. In addition, tracking bone-implanted fiducial markers may still may not provide accurate results for movement or deformation of soft tissue targets. Moreover, whether the fiducial marker is implanted in the bone or injected through a biopsy needle into soft tissue in the vicinity of the target area under computerized tomography (CT) monitoring, the patient must still undergo such invasive procedures before radiation treatment.
A conventional technique that tracks the motion of a tumor without the use of implanted fiducial markers is described in A. Schweikard, H Shiomi, J. Adler, Respiration Tracking in Radiosurgery Without Fiducials, Int J Medical Robotics and Computer Assisted Surgery, January 2005, 19-27. The described fiducial-less tracking technique use image registration methods. These methods may differ depending on the nature of the transformation involved. In particular, the transformation can be rigid or deformable. While rigid transformations (e.g., for head images) typically allow only translations and rotations, deformable transformations require solving a significantly more complex problem.
Image registration methods can be also divided into monomodal (or intramodality) registration and multimodal (or intermodality) registration. In monomodal applications, the images to be registered belong to the same modality, as opposed to multimodal applications where the images to be registered stem from different modalities. Because of the high degree of similarity between the images of the same modality, solving the monomodal registration is usually an order of magnitude easier than in the multimodality applications, especially for deformable transformation.
An existing approach for measuring the patient position and orientation during radiation treatment involves registering projection X-rays taken during treatment with a pre-treatment CT scan. However, this approach is limited because X-rays cannot be taken frequently without additional radiation exposure to the patient. Furthermore, it is difficult to track soft tissue organs (e.g., lungs) on X-rays, without implanting fiducial markers in the vicinity of the target area.